[wolf4ood]

What this PR changes/adds

Adds "Securing the Signaling API" to the data plane page for adopters:

• the data plane's calls to the signaling API are authenticated only when the data plane registered with an
  authorization profile;
• without a profile, any client that reaches the signaling API and knows a transfer process id can change its state;
• expose the signaling API only on the internal network, and use an authorization profile when it is reachable from
  outside that network or shared by several participant contexts.

Why it does that

Data planes without an authorization profile are allowed for deployments where the control plane and the data planes
share a trusted network; the trade-off should be documented.

Further notes

List other areas of code that have changed but are not necessarily linked to the main feature. This could be method
signature changes, package declarations, bugs that were encountered and were fixed inline, etc.

Who will sponsor this feature?

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Linked Issue(s)

Relates to eclipse-edc/Connector#6005

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